Magnetic disk medium, magnetic disk drive, and method for detecting rotational position of magnetic disk medium

ABSTRACT

Embodiments of the invention reduce the information (storage) overhead for determining the rotational position of a magnetic disk medium, as well as reducing the rotational position detection time. In one embodiment, a sector servo magnetic disk medium is configured such that: each track is divided into blocks each including a series of sectors; each block stores a positional information bit string as rotational position information; the positional information bit string is made up of a position code and an identification code, the position code including a block number, the identification code being used to identify the start or the end of the position code; and each bit of the positional information bit string is stored in a different sector of the block. When each block is made up of 12 sectors, for instance, if the block number is denoted as b 0 b 1 b 2 b 3 b 4 b 5 , the positional information bit string may be expressed as 00001b 0 b 1 b 2 1b 3 b 4 b 5 , where the first 5 bits “00001” constitute an identification code.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No.JP2004-338213, filed Nov. 22, 2004, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic disk medium, a magnetic diskdrive, and a method for detecting the rotational position of a magneticdisk medium, and more particularly to a technique of accuratelydetermining the rotational position of a magnetic disk medium with a lowoverhead.

Magnetic disk drives write servo data to their magnetic disk media so asto be able to servo-control their heads. One technique for writing servodata is the sector servo system in which servo data is written to eachsector, or data region, which is a subdivision of a track on a magneticdisk medium. Each piece of servo data includes a track number and asector number.

Conventionally, the rotational position of a magnetic disk medium isdetermined based on a sector number read by the head.

BRIEF SUMMARY OF THE INVENTION

The number of sectors included in each track increases with increasingrecording density of the magnetic disk medium, resulting in an increasein the number of bits required for representing each sector number.Therefore, if the sector number is included in the servo data of eachsector, the area to which user data is written must be reduced, thuscausing the problem of increased overhead.

To address the above problem of increased overhead, one method fordetecting the rotational position of a magnetic disk medium stores anindex bit in a selected sector in each track and counts sectors from theposition of the index bit. However, to detect the index bit of thetarget track under the head, this method generally requires the lengthof time equivalent to one rotation of the magnetic disk medium at themaximum. Furthermore, if the magnetic disk drive has failed to properlydetect the index bit due to external noise or a defect in the magneticdisk medium, it must wait for another rotation, further consuming time.Thus, this method takes a long time to determine the rotational positionof the disk and access a desired sector.

To address the above problem, U.S. Pat. No. 6,327,105 discloses atechnique in which a plurality of codes are associated with differentlocations on each track. The codes are formed such that they can beidentified even when some of their bits are erroneous. The bits of eachcode are written to a series of sectors, one bit for each sector, fromthe associated location. With this technique, however, only specialcodes can be used, limiting the number of usable codes which can bediscriminated from each other. This means that the number of rotationalpositions on each track which can be identified by use of these codes isalso limited, relatively increasing the distance between theserotational positions. Therefore, this technique has a problem in that ittakes a relatively long time to detect the disk rotational position atthe target track under the head, even though the overhead can bereduced.

The present invention has been devised to solve the above problems. Itis, therefore, a feature of the present invention to provide a magneticdisk medium, a magnetic disk drive, and a method for detecting therotational position of a magnetic disk medium, capable of reducing thestorage overhead necessary to detect the rotational position of themagnetic disk medium, as well as reducing the time it takes to detectthe rotational position.

One aspect of the present invention provides a sector servo magneticdisk medium having tracks formed thereon, each track being divided intoa plurality of sectors to which servo data is written, wherein eachtrack is also divided into a plurality of blocks each including one ormore of the plurality of sectors, the one or more sectors beingsequentially arranged along each track, wherein each block stores apositional information bit string which includes address informationcorresponding to a position of each block on the track, wherein thepositional information bit string is made up of a position code and anidentification code, the position code including the addressinformation, the identification code being used to identify the start orthe end of the position code, and wherein the positional information bitstring is divided into a plurality of portions each stored in adifferent sector of the block, each portion forming a predetermined partof servo data stored in the different sector.

Another aspect of the present invention provides a sector servo magneticdisk drive comprising: a magnetic disk medium having tracks formedthereon, each track being divided into a plurality of sectors to whichservo data is written; and a control circuit for detecting a rotationalposition of the magnetic disk medium based on the servo data; whereineach track is also divided into a plurality of blocks each including oneor more of the plurality of sectors, the one or more sectors beingsequentially arranged along each track; wherein each block stores apositional information bit string which includes address informationcorresponding to a position of each block on the track; wherein thepositional information bit string is made up of a position code and anidentification code, the position code including the addressinformation, the identification code being used to identify the start orthe end of the position code; wherein the positional information bitstring is divided into a plurality of portions each stored in adifferent sector of the block, each portion forming a predetermined partof servo data stored in the different sector; and wherein the controlcircuit detects the rotational position based on the positionalinformation bit string read from each block.

Still another aspect of the present invention provides a method fordetecting the rotational position of the above sector servo magneticdisk medium, the method comprising the steps of: as the magnetic diskmedium rotates, sequentially reading the value of each bit of thepredetermined portion of the servo data (in each sector) and generatinga cyclic positional information bit string, the predetermined portion ofthe servo data constituting the positional information bit string;detecting the identification code from the cyclic positional informationbit string; detecting the position code from the cyclic positionalinformation bit string based on a position of the identification code;extracting the address information from the position code; and detectingthe rotational position based on the address information (at the readtiming of the position code).

According to the present invention, it is possible to reduce the storageoverhead necessary to determine the (disk) rotational position, as wellas reducing the time it takes to detect the rotational position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the configuration of amagnetic disk drive according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing the data structure of each sector.

FIG. 3 is a schematic diagram showing the structure of a block.

FIG. 4 is a schematic diagram showing a block, in which the structure ofeach sector is depicted in simplified form.

FIG. 5 is a schematic flowchart of rotational position detectionprocessing performed by the read/write circuit.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 is a schematic block diagram showing the configuration of amagnetic disk drive according to an embodiment of the present invention.The magnetic disk drive comprises a magnetic disk medium (or a disk) 2,a spindle motor (SPM) 4, a head element (or a head) 6, a head arm 8, avoice coil motor (VCM) 10, a head amplifier 12, a drive circuit 14, aread/write circuit 16, an interface circuit 18, and a CPU 20.

The magnetic disk drive writes information along a plurality ofconcentric tracks formed on the surface of the disk 2 using a magneticfield. Each track is divided into a plurality of sectors each havingservo data written thereto beforehand.

The disk 2 is rotated by the SPM 4 at high speed. The head 6 is mountedon the tip of the head arm 8. The direction of the head arm 8 is changedby the VCM 10 so as to move the head 6 from one track to another, thusachieving a seek operation. In a data write or data read operation, thehead 6 is positioned close to the surface of the rotating disk 2. In awrite operation, the head 6 forms a magnetization pattern on the surfaceof the disk 2 which varies along the track. In a read operation, on theother hand, the head 6 senses changes in the magnetic field generatedaccording to the magnetization pattern formed on the surface of the disk2. The SPM 4 and the VCM 10 are driven by the drive circuit 14 under thecontrol of the CPU 20.

The head 6 is connected to the read/write circuit 16 through the headamplifier 12. The head amplifier 12 amplifies the electrical signalinput to or output received from the head 6. From the electrical signalreceived from the head 6, the read/write circuit 16 detects servo datawritten to the disk 2, and obtains positional information on the head 6from the servo data. The read/write circuit 16 obtains the track numberand the sector number constituting the positional information on thehead 6 and outputs them to the CPU 20. Furthermore, the read/writecircuit 16 transmits/receives user data to/from the host computer towhich the magnetic disk drive is connected.

In a data write operation, the interface circuit 18 receives a writeaddress and user data from the host computer. The interface circuit 18outputs the write address to the CPU 20 and buffers the user data. TheCPU 20 issues a control instruction to the drive circuit 14 based on thewrite address, so that, for example, the drive circuit 14 drives the VCM10 to move the head 6 to the track corresponding to the write address.Further, the CPU 20 calculates the timing at which the sectorcorresponding to the write address will reach the position of the head 6based on the positional information on the head 6 supplied from theread/write circuit 16. At a timing matching the calculated timing, theread/write circuit 16 reads the user data (which is to be written to thewrite address) from the buffer memory of the interface circuit 18. Theread/write circuit 16 modulates the user data and then writes it to thedisk 2 through the head 6.

In a data read operation, on the other hand, the interface circuit 18receives a read address from the host computer. The interface circuit 18then outputs the read address to the CPU 20. The CPU 20 issues a controlinstruction to the drive circuit 14 based on the read address, so that,for example, the drive circuit 14 drives the VCM 10 to move the head 6to the track corresponding to the read address. The read/write circuit16 obtains positional information on the head 6 from the servo datastored in each sector passing under the head 6 and outputs it to the CPU20. Furthermore, the read/write circuit 16 demodulates the user datastored in the sectors and outputs it to the interface circuit 18. Theinterface circuit 18 buffers the user data. Based on the positionalinformation on the head 6 supplied from the read/write circuit 16, theCPU 20 determines the timing at which the sector corresponding to theread address was read. The interface circuit 18 outputs the user dataread at the determined timing to the host computer from its buffermemory.

A description will be given below of how the read/write circuit 16detects positional information on the head 6. FIG. 2 is a schematicdiagram showing the data structure of each sector. Specifically, thefigure shows various types of regions in a sector. They are arrangedfrom left to right; the leftmost region is the head portion of thesector while the rightmost region the tail portion. That is, referringto the figure, the head 6 scans the sectors from left to right as thedisk 2 rotates. The servo data portion is on the left and is followed bythe user data portion. The servo data portion comprises a Sync field 30,an STM field 32, a TID field 34, an SN field 36, and a burst signalfield 38. A UD field 40, which is sandwiched by this sector and thesubsequent sector constitutes the user data portion.

The Sync field 30 is a region for storing a signal for establishingsynchronization with the clock and adjusting the gain of the headamplifier 12 so as to maintain constant servo data amplitude. The STMfield 32 stores a unique code common to all sectors, indicating thebeginning of the servo information.

The TID field 34 stores a track number. The track at which the head 6 iscurrently located can be determined by reading the TIED field 34 of theservo data. The SN field 36, on the other hand, is a region for storinginformation indicating the rotational position of the disk 2.

Burst signal fields 38-1 to 38-4 store burst signals A to D,respectively, which give information about the position of the head 6relative to the track. The burst signals A to D are used to control theposition of the head 6 such that it is aligned with the track with highprecision.

Conventionally, the SN field stores a sector number for sequentiallynumbering each sector arranged along the track, and therefore the SNfield has a bit length corresponding to the number of sectors in thetrack. In the magnetic disk drive of the present embodiment, however,each SN field 36 of the disk 2 has only one bit. A description will begiven below of a method for detecting the rotational position of thedisk 2 using the SN field 36.

According to this method, each track is divided into blocks eachincluding a series of k number of sectors. For example, the N^(th) blockincludes the (k*N)^(th) sector to the {k*(N+1)−1}^(th) sector. FIG. 3 isa schematic diagram showing the structure of a block. Specifically, thefigure shows primarily the Nth block that includes 12 sectors arrangedalong a track (k=12).

The SN fields of the 12 sectors in a block each store a 1-bit value(referred to as an SN value) which constitutes a 12-bit stringrepresenting positional information. This positional information bitstring includes a block number, which is address information forindicating the position of each block on a track. This addressinformation can be used to detect the rotational position of the disk 2.FIG. 4 is a schematic block diagram showing a block, in which thestructure of each sector is depicted in simplified form. In the figure,each box 50 represents a servo data portion excluding the burst signalfield 38, while each line 52 connecting between boxes 50 represents therest of the sector. Further, symbol a_(i) denotes the SN value of thei^(th) sector in the block.

The above positional information bit string denoted asa₀a₁a₂a₃a₄a₅a₆a₇a₈a₉a₁₀a₁₁ in the figure includes an identification codeand a position code. The first portion of the positional information bitstring constitutes the identification code, which is a bit string havinga predetermined bit pattern, while the other portion forms the positioncode, which is also a bit string. The position code represents a blocknumber (corresponding to address information). The identification code,on the other hand, is used to identify the start position of theposition code and has a predetermined unique bit pattern which does notappear in any position codes.

For example, the identification code may be a fixed (M+1)bit patternwhose first M bits are 0 and the last bit is 1 (delimiter bit), and theremaining (11−M) bits of the positional information bit string may beused as the position code. In this case, each position code is formedsuch that it does not include any “0” bit strings longer than (M−1)bits, that is, each “1” bit is separated from the subsequent “1” bit byless than M “0” bits. To achieve this, some bits of the position codeare fixed at 1. They are called delimiter bits. Such a delimiter bitarrangement prevents the bit pattern of the identification code fromappearing in the position code, allowing the start position of theposition code to be identified by use of the identification code. Thebits of a position code excluding the predetermined delimiter bitsconstitute a block number.

According to a first working example of the embodiment, M=4, that is,the identification code has the bit pattern “00001”. This is expressedas: a₁a₂a₃a₄=00001. In this case, the remaining 7 bits a₅, a₆, a₇, a₈,a₉, a₁₀, and a₁₁ constitute the position code. In this code, the bit a₈is fixed at 1 as a delimiter bit (thereby dividing the position code)and the rest of the bits (that is, a₅, a₆, a₇, a₉, a₁₀, and a₁₁) areused to form a block number. That is, if the block number is a 6 bitstring denoted as b₀b₁b₂b₃b₄b₅, the two 3-bit strings a₅a₆a₇ anda₉a₁₀a₁₁ correspond to the two 3-bit strings b₀b₁b₂ and b₃b₄b₅,respectively. Therefore, the positional information bit string may beexpressed as 00001b₀b₁b₂1b₃b₄b₅.

According to a second working example of the embodiment, M=3, that is,the identification code has the bit pattern “0001”. This is expressedas: a₀a₁a₂a₃=0001. In this case, the remaining 8 bits a₄, a₅, a₆, a₇,a₈, a₉, a₁₀, and a₁₁ constitute the position code. In this code, thebits a₆ and a₉ are set at 1 as delimiter bits (thereby dividing theposition code) and the rest of the bits (that is, a₄, a₅, a₇, a₈, a₁₀,and a₁₁) are used to form a block number. That is, if the block numberis a 6 bit string denoted as b₀b₁b₂b₃b₄b₅, the three 2-bit strings a₄a₅,a₇a₈, and a₁₀a₁₁ correspond to the three 2-bit strings b₀b₁, b₂b₃, andb₄b₅, respectively. Therefore, the positional information bit string maybe expressed as 0001b₀b₁1b₂b₃1b₄b₅.

In the above two examples, since each block number is expressed as a6-bit binary number, a maximum of 64 blocks (i.e., 768 sectors) can beformed on a single track. It should be noted that in the aboveconfiguration in which each block is made up of 12 sectors, if the valueM is set to 3 or 4, the address bit string {b_(i)} (constituting theblock number) in the positional information bit string has the maximumlength. That is, the number of blocks which can be formed on each trackcan be maximized by setting the value M to 3 or 4. However, if thenumber of blocks that must be formed on each track is 32 or less(corresponding to 384 sectors or less), the bit length of the blocknumber can be reduced to 5. In this case, the remaining address bit maybe used as a parity bit to check each read block number.

In the magnetic disk drive of the present embodiment, the disk 2 has theabove-described configuration and the read/write circuit 16 has acontrol circuit function to detect the rotational position of the disk2, as described above. FIG. 5 is a schematic flowchart of the rotationalposition detection processing performed by the read/write circuit 16when the above first working example using the identification code“00001” is employed.

At step S100, the read/write circuit 16 receives a signal read from thedisk 2 by the head 6 and reads servo data from the signal. Theread/write circuit 16 retrieves the 1-bit SN value in the SN field 36 ofthe servo data of each sector sequentially and generates a bit stringmade up of these SN values. This bit string differs from the abovepositional information bit string of each block, since it is formed bysimply concatenating one bit to another without being aware of blocks.This bit string generated by the read/write circuit 16 may include aplurality of positional information bit strings concatenated to oneanother, and hence is referred to as a cyclic positional information bitstring.

In the rotational position detection processing, each time an SN valueis newly retrieved and appended to the cyclic positional information bitstring, a predetermined number of most recently obtained bit values(naturally including the newly retrieved SN value) are extracted fromthe cyclic positional information bit string and processed as a string.Specifically, the read/write circuit 16 holds a predetermined number(e.g., 12) of most recently obtained bit values (SN values) using a siftregister as a sliding window each time an SN value is newly obtained andadded to the cyclic positional information bit string.

At step S105, the read/write circuit 16 monitors whether theidentification code “00001” appears within the cyclic positionalinformation bit string. An exemplary method of performing thismonitoring is as follows. The values (SN values) of the first 5 bits ofthe shift register are retrieved in parallel and checked to see whethereach SN value coincides with the value of the corresponding bit of theidentification code using a logic circuit formed of inverters, ANDcircuits, etc.

If the identification code has been found at step S110, the read/writecircuit 16 determines the 7-bit string following the foundidentification code as a position code (denoted as a₅a₆a₇a₈a₉a₁₀a₁₁) andextracts a block number N from this position code as addressinformation. Specifically, at step S115, the block number N iscalculated based on the position code using the following equation:N=a₅*2⁵+a₆*2⁴+a₇*2³+a₉*2²+a₁₀*2²+a₁₁.

Once the block number N of the block currently passing under the head 6is thus obtained, the sector number of the first sector of the nextblock can be automatically determined as 12*(N+1) and then the sectornumbers of the subsequent sectors can be determined accordingly at stepS120. Thus, the read/write circuit 16 determines these sector numbers asinformation about the rotational position of the disk 2 and outputs themto the CPU 20.

It should be noted that in addition to the above bit string “00001”,examples of identification codes include “11110” (obtained by invertingeach bit of the above bit string), “10000” (obtained by setting thefirst bit to 1 and the remaining 4 bits to 0), and “100001” (obtained bysetting the first and last bits to 1 and the remaining 4 bits to 0).

In the above configuration, each bit of a positional information bitstring is stored in a different sector. This can reduce the storageoverhead necessary to determine the disk rotational position, ascompared to conventional configurations in which each sector stores itssector number.

Further, each track is divided into a relatively large number of blocks(for example, 64 blocks) and the rotational position of the disk 2 isdetected each time the disk 2 rotates by an angle equivalent to oneblock, which reduces the time it takes to determine the rotationalposition. That is, the time required to determine the rotationalposition decreases with increasing number of blocks formed on eachtrack. Therefore, the number of blocks to be formed on each track may bedetermined based on the maximum number of blocks that can be representedby using the bits of the address bit string {b_(i)} included in thepositional information bit string, which is effective in reducing therotational position detection time. Specifically, the number of blocksmay be set based on the largest block number which can be expressed bythe address bit string.

It should be noted that some overhead reduction effect can be obtainedif the bit length of the portion of the positional information bitstring stored in each sector is smaller than the bit length required torepresent each sector number. Therefore, the positional information bitstring may be divided into 2-bit strings and each 2-bit string may bestored in a different sector.

Further, the position code may represent address information other thanthe block number. For example, each position code may be formed suchthat it indicates the sector number of the first sector or the lastsector of a block as address information. In such a case, however, sincethe sector number has a larger bit length than the block number, thelength of the positional information bit string must be adjustedaccordingly.

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reviewing the above description. Thescope of the invention should, therefore, be determined not withreference to the above description, but instead should be determinedwith reference to the appended claims alone with their full scope ofequivalents.

1. A sector servo magnetic disk medium having tracks formed thereon,each track being divided into a plurality of sectors to which servo datais written, wherein each track is also divided into a plurality ofblocks each including one or more of said plurality of sectors, said oneor more sectors being sequentially arranged along said each track,wherein each block stores a positional information bit string whichincludes address information corresponding to a position of said eachblock on the track, wherein said positional information bit string ismade up of a position code and an identification code, said positioncode including said address information, said identification code beingused to identify the start or the end of said position code, and whereinsaid positional information bit string is divided into a plurality ofportions each stored in a different sector of the block, said eachportion forming a predetermined part of servo data stored in saiddifferent sector.
 2. The sector servo magnetic disk medium as claimed inclaim 1, wherein said portions of said positional information bit stringeach consist of one bit.
 3. The sector servo magnetic disk medium asclaimed in claim 1, wherein: said identification code includes a stringof M bits each having a first bit value and further includes a bithaving a second bit value which immediately precedes or succeeds saidstring (where M denotes an integer equal to or larger than 2); and saidposition code includes a plurality of strings each having a length lessthan M bits and further includes bits each having said second bit value,said plurality of strings being separated from one another by one ofsaid bits having said second bit value, said plurality of stringscollectively forming an address bit string corresponding to said addressinformation.
 4. The sector servo magnetic disk medium as claimed inclaim 3, wherein said address bit string is a binary block number forsequentially numbering each block on a track.
 5. The sector servomagnetic disk medium as claimed in claim 4, wherein when the proportionof said address bit string in said positional information bit stringstored in each block is maximized, the number of blocks on each track isset based on the largest block number that can be expressed by saidaddress bit string.
 6. The sector servo magnetic disk medium as claimedin claim 5, wherein: said portions of said positional information bitstring each consist of one bit; each block includes 12 sectors; and saidvalue M is 3 or
 4. 7. The sector servo magnetic disk medium as claimedin claim 6, wherein: said identification code includes a string of Mbits each having a value of 0 and further includes a bit having a valueof 1 which immediately follows said string; said position codeimmediately follows said identification code; and said position codeincludes strings each having a length of (M−1) bits and further includesbits each having a value of 1, said strings being separated from oneanother by one of said bits having a value of 1, said stringscollectively forming said address bit string.
 8. A sector servo magneticdisk drive comprising: a magnetic disk medium having tracks formedthereon, each track being divided into a plurality of sectors to whichservo data is written; and a control circuit to detect a rotationalposition of said magnetic disk medium based on said servo data; whereineach track is also divided into a plurality of blocks each including oneor more of said plurality of sectors, said one or more sectors beingsequentially arranged along said each track; wherein each block stores apositional information bit string which includes address informationcorresponding to a position of said each block on the track; whereinsaid positional information bit string is made up of a position code andan identification code, said position code including said addressinformation, said identification code being used to identify the startor the end of said position code; wherein said positional informationbit string is divided into a plurality of portions each stored in adifferent sector of the block, said each portion forming a predeterminedpart of servo data stored in said different sector; and wherein saidcontrol circuit detects said rotational position based on saidpositional information bit string read from each block.
 9. The sectorservo magnetic disk drive as claimed in claim 8, wherein said portionsof said positional information bit string each consist of one bit. 10.The sector servo magnetic disk drive as claimed in claim 8, wherein:said identification code includes a string of M bits each having a firstbit value and further includes a bit having a second bit value whichimmediately precedes or succeeds said string (where M denotes an integerequal to or larger than 2); and said position code includes a pluralityof strings each having a length less than M bits and further includesbits each having said second bit value, said plurality of strings beingseparated from one another by one of said bits having said second bitvalue, said plurality of strings collectively forming an address bitstring corresponding to said address information.
 11. The sector servomagnetic disk drive as claimed in claim 10, wherein said address bitstring is a binary block number for sequentially numbering each block ona track.
 12. The sector servo magnetic disk drive as claimed in claim11, wherein when the proportion of said address bit string in saidpositional information bit string stored in each block is maximized, thenumber of blocks on each track is set based on the largest block numberthat can be expressed by said address bit string.
 13. The sector servomagnetic disk drive as claimed in claim 12, wherein: said portions ofsaid positional information bit string each consist of one bit; eachblock includes 12 sectors; and said value M is 3 or
 4. 14. The sectorservo magnetic disk drive as claimed in claim 13, wherein: saididentification code includes a string of M bits each having a value of 0and further includes a bit having a value of 1 which immediately followssaid string; and said position code immediately follows saididentification code; and said position code includes strings each havinga length of (M−1) bits and further includes bits each having a value of1, said strings being separated from one another by one of said bitshaving a value of 1, said strings collectively forming said address bitstring.
 15. A method for detecting a rotational position of a sectorservo magnetic disk medium having tracks formed thereon, each trackbeing divided into a plurality of sectors to which servo data iswritten, wherein each track is also divided into a plurality of blockseach including one or more of said plurality of sectors, said one ormore sectors being sequentially arranged along said each track, whereineach block stores a positional information bit string which includesaddress information corresponding to a position of said each block onthe track, wherein said positional information bit string is made up ofa position code and an identification code, said position code includingsaid address information, said identification code being used toidentify the start or the end of said position code, wherein saidpositional information bit string is divided into a plurality ofportions each stored in a different sector of the block, said eachportion forming a predetermined part of servo data stored in saiddifferent sector, and wherein said method comprises: as said magneticdisk medium rotates, sequentially reading the value of each bit of saidpredetermined portion of said servo data in each sector and generating acyclic positional information bit string, said predetermined portion ofsaid servo data constituting said positional information bit string;detecting said identification code from said cyclic positionalinformation bit string; detecting said position code from said cyclicpositional information bit string based on a position of saididentification code; extracting said address information from saidposition code; and detecting said rotational position based on saidaddress information at the read timing of said position code.
 16. Themethod as claimed in claim 15, wherein said portions of said positionalinformation bit string each consist of one bit.
 17. The method asclaimed in claim 15, wherein: said identification code includes a stringof M bits each having a first bit value and further includes a bithaving a second bit value which immediately precedes or succeeds saidstring (where M denotes an integer equal to or larger than 2); and saidposition code includes a plurality of strings each having a length lessthan M bits and further includes bits each having said second bit value,said plurality of strings being separated from one another by one ofsaid bits having said second bit value, said plurality of stringscollectively forming an address bit string corresponding to said addressinformation.
 18. The method as claimed in claim 17, wherein said addressbit string is a binary block number for sequentially numbering eachblock on a track.
 19. The method as claimed in claim 18, wherein whenthe proportion of said address bit string in said positional informationbit string stored in each block is maximized, the number of blocks oneach track is set based on the largest block number that can beexpressed by said address bit string.
 20. The method as claimed in claim19, wherein: said portions of said positional information bit stringeach consist of one bit; each block includes 12 sectors; and said valueM is 3 or
 4. 21. The method as claimed in claim 20, wherein: saididentification code includes a string of M bits each having a value of 0and further includes a bit having a value of 1 which immediately followssaid string; said position code immediately follows said identificationcode; and said position code includes strings each having a length of(M−1) bits and further includes bits each having a value of 1, saidstrings being separated from one another by one of said bits having avalue of 1, said strings collectively forming said address bit string.